Adsorption-Induced Forces

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

We shall note that there are numerous experimental papers dealing with studies of effect of adsorption on other electrophysical characteristics of oxide adsorbents such as thermal electromotive force [107], Hall effect [108 - 110], volt-ampere [58, 111] and frequency [112] characteristics. The availability of results of these studies makes one expect that in near future the adequate theoretical model describing adsorption-induced changes in electrophysical characteristics of semiconductor adsorbent will be developed. 

The main contributions to AadsG for a globular protein are from electrostatic, dispersion, and hydrophobic forces and from changes in the structure of the protein molecule. Although in this section these contributions are discussed individually, strict separation of the influence of these forces on the overall adsorption process of a protein is not possible. For instance, adsorption-induced alteration of the protein structure affects the electrostatic and hydrophobic interaction between the protein and the surface. When the sorbent surface is not smooth but is covered with (polymeric)... 

Sato, T., and Ruch, R., Stabilization of Colloidal Dispersions by Polymer Adsorption, Marcel Dekker, New York, 1980. (Research monograph. An advanced treatment of polymer-induced forces.)... 

In the context of this discussion, surface heterogeneity will be expressed in terms of the adsorptive potential of the material. The adsorptive potential is a measure of the net attraction between a solid surface and an adsorbed probe molecule. For physical adsorption, these forces arise chiefly from London-type dispersion interactions (van der Waals forces) resulting from induced-dipole/induced-dipole and higher multipolar attractions which in turn depend on the size. 

In the discussion of the reversibility of the Rehbinder effect, it was implied that there is a thermodynamically stable interface present between the mutually saturated solid phase and liquid medium and that the effect vanishes when the liquid medium is ranoved, for example, by evaporation. These two peculiarities make the Rehbinder effect principally different from the corrosion caused by the action of aggressive media. At the same time, one must realize that complete segregation is not possible various processes can cover a fairly broad spectrum from idealized cases involving purely mechanical failure to purely corrosive processes (or dissolution). The Rehbinder effect, which involves the adsorption-induced lowering of strength, stress-facilitated corrosion, and corrosive fatigue, often occupies intermediate positions in these series. In this type of phenomenon, the action of external forces and the action of chemically active media both contribute to the net result in certain proportions. 

In many applications in mineral processing, papermaking, and waste water treatment, the opposite effect is desired. Then, polymers are used to induce flocculation. Usually, this is achieved by adding polymer, which can bridge two particles. To understand all these effects, we first need to introduce some fundamentals of the structure of polymers in solution and of polymer adsorption. Good introductions into polymer physics are Refs [1308-1310]. Polymer-induced forces are reviewed in Refs [1311 -1313]. 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

The importance of surface characterization in molecular architecture chemistry and engineering is obvious. Solid surfaces are becoming essential building blocks for constructing molecular architectures, as demonstrated in self-assembled monolayer formation [6] and alternate layer-by-layer adsorption [7]. Surface-induced structuring of liqnids is also well-known [8,9], which has implications for micro- and nano-technologies (i.e., liqnid crystal displays and micromachines). The virtue of the force measurement has been demonstrated, for example, in our report on novel molecular architectures (alcohol clusters) at solid-liquid interfaces [10]. 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

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